Ballast apparatus for operating fluorescent lamps and electrical coil assemblies therefor



March 22, 1966 P. w. DAVIS, JR 3,242,381

BALLAST APPARATUS FOR OPERATING FLUORESCENT LAMPS AND ELECTRICAL COILASSEMBLIES THEREFOR 4 Sheets-Sheet 1 Filed Jan. 2, 1963 INVENTOR.

Pau/ WDawlsJn 7& M

ATTORNEY March 22, 1966 p, w s, JR. 3,242,381

BALLAST APPARATUS FOR OPERATING FLUORESCENT LAMPS AND ELECTRICAL COILASSEMBLIES THEREFOR Filed Jan. 2, 1963 4 Sheets-Sheet 2 E lrs 5 45 40 o'0 g T 2 Al ii s 5 47 52- 49 s3 l OVEN f0 I I 55 i j; \A L2 5|- L50 42L| R I s R W l 46 OSCILLOSCOPE R\ ElE E BEAT FREQUENCY OSCILLATOR 66 R40 C YQIVA'AVA'AVA 4 A INVENTOR. 1%41/ MZDaV/s-In ATTORNEY March 22. 1966P. w. DAVIS. JR 3,242,331

BALLAST APPARATUS FOR OPERATING FLUORESCENT LAMPS Filed Jan. 2; 1963 ANDELECTRICAL COIL ASSEMBLIES THEREFOR 4 Sheets-Sheet 3 o o o 160 2 00TIME- MINUTES IOO INVENTOR. Pau/ Wfia |//$,Jr: 7% 77am ris 26oTEMPERATURE-CENTIGRADE .OOl

ATTORNEY March 22. 1966 P. w. DAVIS. JR 3,242,331

BALLAST APPARATUS FOR OPERATING FLUORESCENT LAMPS AND ELECTRICAL COILASSEMBLIES THEREFOR Filed Jan. 2, 1963 4 Sheets-Sheet A.

INVENTOR. Pau/WDaWSJn BY 7 147 74M ATTORN EY United States Patent 3 242381 BALLAST APPARATUS oR OPERATING FLUO- RESCENT'LAMPS AND ELECTRICALCOIL AS- SEMBLIES THEREFOR 7 Paul W.'Davis, Jr., Danville, Ill.,assignor'to General Electric Company, a corporation of New York FiledJan. 2, 1963,Ser'. No. 249,099

8 Claims. 1 (Cl. 315-487) This invention relates to ballast apparatusand to improved electrical coil assemblies for use in ballast apparatusfor operating fluorescent lamps.

In electrical coils used in ballast transformers layer insulation isgenerally provided between the layers of conductors. Such convention-a1coils are wound with aninsulated conductor wire and are not usuallycompletely impregnated in applications where the open circuit voltage isless' than 300 volts. For'example, a representative electrical coil usedin a low voltage fluorescent lamp ballast having and open circuitvoltage of less than 300 volts is Wound with conductor wire insulatedwith a thin nylon coating and with layer insulation formed of vegetableparchment orpaper approximately .002 of an inch in thickness. Theconductor wire is wound over the layer insulation so that the layerinsulation is interleaved between the layers of conductor wire. Thus,the layers ofconductor wire are not contiguous. The voltage betweenlayers in such a typical ballast coil is inthe neighborhood of ten voltsand between adjacent turns-the voltage is'abo ut one tenth of'a volt.

The paper layer insulation in such coils minimizes the effect of voltagestress between conductor .wire layers by pioviding'an insulating barriertherebetween. Since electrical coils used in ballasts are Wound onspools withoutrirns, the layer insulation also serves to prevent the endturns of a coil from being mechanically displaced.

The electrical coil or coils used in a ballast transformer o'rreactorare generally disposed on a central winding le'g within a coil receivingWindow defined by the center winding leg and side yoke members whichprovide a return path for the magnetic flux. Generally, the length ofthe ballast coil dictates the length of the coil receiving windowsof'the magnetic core of the ballast transformer since thecross-sectional dimensions are more or less fixed by the requirementthat the ballast case not exceed certain specified dimensions in orderthat it can be mounted in alamp 'fixture.

It will be appreciated that the layer insulation used in conventionalballast coils adds to the over-all coil volume. A coil with layerinsulation, as the term is used herein, denotes an electrical coil withflexible sheet insulation, such as paper, interleaved between the layersof conductors. Several types of coils are not wound with layerinsulation between the layers of conductor wire. In a precision woundcoil, for instance, each layer of turns is formed of consecutivelywoun'd turns which are accurately positioned to prevent any fall-throughof a turn to an adjacent layer. A random wound coil is also woundwithout layer insulation but, as the term implies, the coil iswound-without any special provision being made for insuring that eachturn of the conductor wire will fall in its proper layer. Consequently,in a random wound coil a turn may be displaced one or more layers fromits normal layer position or the position it would have occupied if thecoil were precision wound.

As comparedwith c-oils not employing layer insulation but having thesame number of turns, a conventional ballast coil with'layer insulationwill occupy more window space in -a shell-type of ballast transformer.With the dimensional limitations imposed on a ballast, it is necessarythat the length of the core be increased to provide the necessary windowspace to accommodate a longer coil. The use of electrical coils withlayer insulation has RF i 1C6 therefore made it necessary to use largerferromagnetic cores and larger ballast cases. Consequently, ballastcoils with layer insulation do not result in the most economicalutilization of materials. Also, fluorescent lamp ballasts usingelectrical coils employing layer insulation are more costlyttomanufacture since the layers of magnet wire must be wound over thelayers of insulating parchment. Accordingly, there has been a longstanding need for ballasts that can employ coils without paper layerinsulation.

Since ballasts operating fluorescent lamps arerequired to have anaverage continuous service life over a period of approximately twelveyears, the expected life of a ballast or ballast apparatus is usuallydetermined by temperature accelerated life tests inorder to achieve areasonably accurate estimate of expected service life in a relativelyshort period of time. In conducting such tests, coil samples areassembled in ballasts, andthe ballasts are then operated under normalcurrent and voltage'conditions in an elevated ambient'temperatureprovided by a circulating air oven.

chemical reactions and since the rate of a given chemical reaction canbe determined as a function of temperature; it is possible to selectelevated temperatures and shortened periods of time to determine therate at which the reactions will occur at normal operating temperaturesand thereby estimate the service life of the ballast. A

conventional coil using a magnet wire coated with nylon and having alayer insulation consisting of vegetable parchment 0.002 inch inthickness has an expected service life of 12 years based on 5000 hoursof operation per year at a maximum average coil temperature of degreescentigrade.

By way of comparison, two groups of ballasts employing coils randomwound with identical magnet wire' coated with nylon were life tested.Ten of the coils in one group were vacuum impregnated with a mixture ofa synthetic fatty acid amide type wax and asphalt.

The coils of the other group were not impregnated."

least 3750 hours of operation at the elevated temperature. Unless theballast meets this requirement, it cannot be expected to provide anexpected service life of 12 years based on 5000 hours of operation peryear at a maximum coil temperature of 105 degrees centigrade.

In the past, it was generally believed that a primary factor in thepremature failure of coils not employing layer insulation in ballastsundergoing temperature accelerated life tests was internal shortingresulting from This" a copper-to-copper type of contact between turns.copper-to-copper type of contact between turns was also generallybelieved responsible for coil failures in-ballasts installed in lampfixtures. Various theories have been advanced to explain the mechanismof this copper-tocopper type of contact. For example, it'has beengenerally assumed that the copper-to-copper contact in a coil notemploying layer insulation is the result of a number of factors, such ascut-through due to plastic flow of the Wire coating at points of maximummechanical stress or the alignment of the various breaks in theinsulating coating on the magnet wire. These cutthroughs and breaks canbe variously caused by bending the magnet wire over small radius bobbincorners, by winding friction, by careless handling and other similarcauses. A common explanation has been that the breaks in the insulatingcoating result in a short circuit of the Since the degradation oforganic insulating materials can be treated mathematically as a seriesof coil thereby initiating the degradation processes which prematurelycause the coil to fail.

Heretofore, the tests carried out to determine the suitability of coilsnot employing layer insulation such as are used in ballast transformerswere directed towards a determination of the susceptibility of themagnet wire enamel to breaks and cut-throughs. Usually, such tests asabrasion resistance, flexibility, and cut-through temperature tests havebeen employed. Although coils utilizing conductor wire satisfactorilymeeting the requirements of such tests have been tested in ballasts, theballasts still could not satisfactorily pass temperature acceleratedlife tests.

From the foregoing considerations, it will be apparent that there is aneed for an improved ballast employing electrical coils that do notrequire the use of layer insulation and that can without such layerinsulation satisfactorily meet the requirements of temperatureaccelerated life tests. Further, it is evident that such an arrangementthat does not require layer insulation will result in benefits asreduced ballast size and weight, and in a more economical utilization ofmaterials.

Accordingly, it is a general object of the invention to provide animproved fluorescent lamp ballast employing coils without layerinsulation.

Another object of the present invention is to provide an improved coilassembly for use in ballasts for operating fluorescent lamps.

It is another object of the invention to provide an improved coilwithout layer insulation for use in a shell type of transformer used tooperate fluorescent lamps.

It is a further object of the present invention to provide an improvedballast apparatus wherein the size of the ballast is reduced as comparedwith similar ballasts for operating comparable fluorescent lamps.

A more specific object of the present invention is to provide a new andimproved ballast wherein the amount of material required to carry outthe voltage transforming and current limiting functions of the ballastis appreciably reduced as compared with similar ballasts used foroperating comparable lamps.

In accordance with one form of my invention, I have provided an improvedballast apparatus for operating one or more fluorescent lamps from analternating power source in which the ballast transformer includes acoil assembly having at least a secondary winding comprised of layers ofconductor wire without layer insulation, such as paper, interposedbetween the layers of conductor wire. The ballast transformer alsoincludes a primary winding inductively coupled with the secondarywinding on a magnetic core.

I have found that it is possible to employ an electrical coil in a shelltype of ballast transformer for operating fluorescent lamps without needfor layer insulation if the electrical parameters of the equivalentcircuit of the coil are such that the power dissipated per unit volumeof the insulating enamel on the conductor wire of the coil and per unitweight of the metallic conductor of the wire is maintained below aspecified limit. Thus, a coil without layer insulation may be used in aballast for operating fluorescent lamps if the electrical coil isprovided with an equivalent circuit resistance R and an operatingvoltage V, such that the power in watts dissipated in the equivalentcircuit divided by the number of cubic centimeters of insulating enamelin the coil and divided by the number of kilograms of metallic conductorin the coil is less than 1.6 at a frequency of 60 cycles per second andat a temperature between 140 and 180 degrees centigrade.

The subject matter which I regard as my invention is set forth in theappended claims. The invention itself, however, together with furtherobjects and advantages thereof may be better understood by referring tothe following description taken in connection with the accompanyingdrawings in which:

FIGURE 1 is an illustration of a coil equivalent circuit;

FIGURE 2 is a sectionalized View showing an idealized conductor wirelayer arrangement of a precision wound coil;

FIGURE 3 is a sectionalized view illustrating random wound coils inwhich the conductor wire has a two layer fall-through or is displacedtwo layers from its natural position;

FIGURE 4 represents a sectionalized view of an idealized bifilar woundcoil illustrating the normal disposition of conductor wires in such acoil;

FIGURE 5 is a schematic circuit diagram of a test circuit used to makedynamic measurements of the electrical performance of the bifilar woundcoil samples used to simulate precision and random wound ballast coils;

FIGURE 6 represents a plot of time in minutes versus coil temperaturefor representative bifilar coils wound with conductor wire coated withvarious wire enamels;

FIGURE 7 represents a plot of temperature in degrees centrigrade versuspower density in the coil dielectric material in watts per gram forbifilar coils with conductor wire coated with various wire enamels;

FIGURE 8 is a schematic circuit diagram of the apparatus used todetermine the critical frequency used to compute the equivalent circuitparameter values of the ballast coils;

FIGURE 9 is a diagram illustrating how the reference capacitor used todetermine the equivalent capacitance was arranged;

FIGURE 10 is a perspective view of a ballast embody-' ing the improvedcoil arrangement of one form of the invention with a part of the caseand coil cut-away;

FIGURE 11 is a plan view of the ballast transformer shown in FIGURE 10;

FIGURE 12 is a schematic circuit diagram of a ballast apparatusincorporating the ballast transformer shown in FIGURES 10 and 11; and

FIGURE 13 illustrates a sectionalized view of a conventional highvoltage ballast coil assembly.

As is shown in FIGURE 1, an electrical coil may be represented by anequivalent circuit consisting of a resistor with an equivalentresistance R, an inductor with equivalent inductance L and a capacitorwith an equivalent capacitance C. It will be seen that the resistor,inductor and capacitor are connected in parallel circuit relation witheach other. As will hereinafter be more fully explained, the equivalentresistance R at an elevated temperature is a significant parameter indetermining the power dissipated in the wire insulating enamel of a coilat the elevated temperature and is an important factor in deter-'insulation when used in fluorescent lamp ballasts. In-

stead I have discovered that a thermal runaway mechanism is a key factorin the failure mechanism of such electrical coils when used in ballastsfor operating fluorescent lamps.

In temperature accelerated life tests carried out to investigate thefailure mechanism of ballast coils, the coil surface temperatures weremeasured by means of thermocouples taped to the outside surfaces of thecoils. In most cases, I observed that the ultimate failure of theballast coil without layer insulation was preceded by a period ofseveral hours during which the coil temperatures. steadily rose tolevels as high as degrees above the nominal coil temperatures of todegrees centi grade.

Since the ballast coils without layer insulation which were examined forbreaks after finishing and the coils with-- out layer insulation whichhad been heat-aged for peri-- ods longer than the lifetime of similarcoils operating at the same or lower temperatures in temperature accel--erated life tests did not show any evidence of QOPEI-to-coppercontacts', it was considered'improbable that ductor wirespositioned side by side in each layer, can

be used to simulate the electrical conditions of a random or precisionwound coil. With a bifilar coil simulating a random or precision woundcoil, it waspossible to more readily "make various electricalmeasurements as will now be more fully explained. I,

Referring to FIGURES 2, 3 and 4', I have shown there-.

in a crosssectional view of a representative center turn and its sixadjacent'turns of a precision, random and bifilar wound coil. It will beappreciated that in the views shown in FIGURES 2, 3' and 4, thediameter'of the con ductor wire has been greatly enlarged relative tothe coil dimensions. Thishas been done to bore clearly show the"relative disposition of the turns In the precision wound coil 10, asshown in FIGURE 2, the center turn 11 is surrounded by and in contactwith six other turns 12, 13, 14; 15, .16 and 17, as will be.

seen' intheview of the lower halflof thecoil 10. Two of the turns 12, 13are inthe inner layer and the two other turns '15', 16*are in the outerlayer. The two adjacents turn 14 'and' 17 are disposed inthe same layeras the center turn 11. It'will be appreciated that the two adjacerrtturns14 and 17are disposed in the same layer as the center turn 11.

If we assume that the voltage difference between layers is V any currentflowing between the center turn 11? and a'turn, such as .12, 13, 15 or16, in the adjacent layers will be equal to the voltage difference Vdivided by the effective resistance R between the center turn 11 and anadjacent turn, It will be understood that the effective resistance R.which is the resistance between adjacent turns, ditfersfrom theequivalent resistance R of the equivalent coil circuit. The equivalentresistance R is.

the pure resistance which when placed electrically in parallel withtheequivalent inductance L and equivalent ca-- pacitance C, as shown inFIGURE 1, will form a combined'circuitthat exhibits an electricalbehavior essentially similar to that of the coil. The effectiveresistance R on theother hand, is the resistance in the area ofcontacfbetween one turn and an adjacent turn.

It was found that in the type of ballast coils which were being, tested,having approximately 100 turns per layer and 13 layers, the voltagedifference between adjacent turns in the same layer couldbeig'noredsince this voltage diiference was approximately of the voltagedifference V between layers Accordingly, for a precision wound coil, thetotal effective current I between the center turnll and the adjacentturns 12, 13, 1'5 and 16-in the inner and outer layers may be expressedas follows:

4 L I RE where'R is the effective resistance in the area of contactbetween the center turn and an adjacent turn in an adjoining layer ofthe coil.

'From an examination of numerous random wound coils wound on a fiyerhead winding machine, it was found that the maximum displacement of awire from its normal position was usually two layers. Thus, the mosthighly stressed turns in a random wound coil are assumed to be the turnsthat are displaced two layers from this normal position.

In FIGURE 3, I have illustrated a group of seven turns in an enlargedcross-section of a random; wound coil 20,

in which a center turn 21 is assumed to be displaced inwardly twolayers. I have indicated inthe upper sectionalized view of the coil 20the idealized voltage relationships between the centerturn 21 and sixadjacentturns 22, 23, 24, 25, 26 and 27.

Assuming that a potential or voltage difference-V ex'-- ists between thecenter turn- 21 and the two innerturns 22, 23, the voltage differencebetween the center turn 21 and one of the adjacent turns 24, 27 in thesame layer will be twice the voltage difference between layers or ZVsince the center turn is displaced two layers from its normal position.Further, it will be seen that the voltage difference between the centerturn 21 and each of the outer adjacent turns 25 and 26 will beapproximately equal to three times the" voltage differencebetween'layers 'or'3V Accordingly, the effective current I between acenter turn and the adjacent turns may be expressed as follows:

where R is the effective resistancein the areaof contact between thecenter turn and an adjacent turn'in an ad'- joining layer of the coil.

I have found that a bifilar coil can be used tosimulate the electricalconditions of a precision or random wound coil by adjusting the voltagedifference between the conductor wires. If the bifilar coil 30 of FIGURE4 is truly precision wound, a center turn 3l may'be' assumed'to be incontact with six adjacent turns 32, 33, 34, 35, 36 and" 37. Two of theturns, adjacent to thecenterturn- 31,

such as turns 33 and 36, are extensions of the center'turn v 31, and thevoltage difference between these turns is zero when the applied voltageis impressed between the two windings. The voltage difference betweenthe center turn" 31 and turns 32, 34, 35 and 37 may be assumed to be VThus, the effective current I maybe expressed as follows:

I RE where R is the effective resistance in'the contact area between acenter turn and an adjacent turn of the coil.

If it is desired to have the bifilar coil simulate theelectricalconditions of a precision wound coil, the voltage difference V betweenthe bifilar windings should be made" approximately equal to the voltagedifference V between the layers of the precision wound coil. In atypical ballast coil, this voltage difference V was about 10 volts;

Where it is desired to simulate a rand-om woundcoil, the voltagedifference V may be taken as being approximately equal to three timesthe voltage difference V In the random wound bifilar coils tested, aswill hereinafter be morefully described, .a voltage of 30 volts r;m.s.or 3 times the normal layer to layer voltage was impressed across thebifilar windings. The bifilar coils used were wound on a winding latheto the same geometry as the comparable ballast coil. The coil geometrywas closely controlled by winding the coils on phenolic bobbins;

To further investigate the mechanism of coil "failure in a ballasttransformer, an apparatus'40, as shown inFIG- URE 5', was devised todynamically determine-the electrical behavior of the bifilar wound'co-il sample simulating ballast coils under elevated temperatureconditions; The-j particular coils tested were random wound. In order toproduce internal heating within the bifilar windings L L a transformer Twas used. The transformer T include-d a pair of secondary windings S andS inductively coupled with a primary winding P on a magnetic core 41.Since the secondary windings S and S are connected across the bifilarwindings L L it will be understood that the insulation impedance of thesecondary windings S and S should be on the order of several magnitudesgreater than the insulation impedance of the bifilar wind- .provided inthe secondary circuits so that the normal ballast operating currentdensity in the copper of the bifilar windings L L could be maintianed.The terminal leads 42 and 43 of the primary winding P were connected toa- 120 volt 60 cycle power supply.

Since the bifilar windings L and L were not disposed on a magnetic core,the voltage required to produce the required current density was about 3volts. Since this voltage stress is not of the magnitude that wouldnormally be encountered between turns in a ballast coil, a second.transformer circuit including a transformer T and a variableautotransformer T was provided. The transformer T was an isolationtransformer having a secondary winding S and a primary winding Pinductively coupled on a magnetic core 42. The autotransformer Tincluded an autotransformer winding A and an adjustable tap 45 toprovide a variable voltage output across the autotransformer winding AOne end of the autotransformer winding A was connected with winding Land the other end was connected with winding L through a resistor RThus, the voltage supplied across the bifilar windings L and L could bevaried to supply a voltage stress that was comparable to the voltagestress normally encountered in a ballast coil.

Leads 46 and 47 were connected to the vertical amplifier terminals 48,49 of an oscilloscope 50 schematically shown enclosed in the dashedrectangle. These electrical connections were made so that a verticaldeflection proportional to the voltage drop across the resistor R or, inother L was produced on the oscilloscope 50. Leads 51 and 52 wereconnected in circuit with the horizontal amplifier terminal 53 and 54 sothat a horizontal deflection proportional to the voltgae drop across theresistor R or, in other words, proportional to the current flow betweenthe bifilar windings L and L was produced. It will be noted that currentflowing between the bifilar windings L and L will produce a current flowthrough the resistor R It was found that with a ohm ressitor R a currentof approximately 1X10 amperes in magnitude produced a deflection of 1centimeter when the one millivolt per centimeter range of the horizontalamplifier of the oscilloscope 50 was employed. The phase angle of thevoltage impressed across the bifilar windings L and L and the currentbetween them was measured by using a Hewlett-Packard Webb phase-shiftmask placed on the face of the cathode ray tube of the oscilloscope 50.With this device it was possible to read the phase angle within plus orminus 1 degree of arc. An ammeter 55 was connected in circuit with thewinding L and also an ammeter 56 was connected in circuit with winding Lso that the current flow through the windings could be observed.

Before a bifilar coil containing the windings L and L was placed in theoven 60, a thermocouple was taped to the outer surface of the coil.After the thermocouple was attached, the bifilar coil was placed in a250 milliliter glass beaker, and the beaker was filled with an asphalticpotting material containing 48 percent by weight of blown petroleumasphalt having a 118 degree centigrade softening point and containing 52percent by weight of silica. The glass beaker was filled with theasphaltic potting material to a height suflicient to completely coverthe coil.

Since in a ballast transformer approximately half of the heat comes fromlosses in the steel of the magnetic core, this heating effect wassimulated by placing the coil in the forced air oven 60. In order tosimulate the poor heat transfer away from the interior of a ballast andalso to reduce the rapid cooling resulting from air flow in the oven,the beaker containing the bifilar coil was placed in a cardboardcontainer, the glass beaker was then covered with chopped glass rovingsto a depth of about A of an inch and several straps of mylar tape wereplaced across the top of the cardboard container in order to retain thechopped glass rovings.

. Readings were taken of current, voltage, phase angle and coiltemperature over temperatures ranging from degrees to 170 degreescentigrade. In FIGURE 6, I have shown a plot of coil temperature indegrees centigrade versus time in minutes for unimpregnated coilswithout layer insulation wound with a single film build nylon wire,curve A, and a single film build Formex wire, curve B. It will be seenfrom the sharp rise in curve A representing the nylon insulated magnetwire that a source of heat is present in the nylon wound bifilar coilthat was not present in the coil wound with the Formex wire.

The conductor wire identified herein by the registered trademark Formexemploys an insulation coating that is the reaction product of apartially or completely hydrolyzed polymerized vinyl ester and analdehyde. The insulation is more fully described in US. Letter PatentNo. 2,085,995 granted to W. I. Patnode et al. The trademark Formex isused herein for the purpose of conveniently identifying the wire used.

In FIGURE 7, I have illustrated a plot of the power density in the wirecoating in watts per gram versus the coil temperature in degreescentigrade as determined from thermocouple readings. In making theseplots it was assumed that the measured power was being dissipated in thewire coating. Curve A represents the values obtained for the simulatedrandom wound coil with a nylon wire and curve B represents the valuesobtained for a simulated random wound coil with the Formex wirecorresponding to the curves A and B of FIGURE 6. Curve C represents aplot of the values of power density against temperature for a bifilarcoil wound with Formex wire with an asphalt base impregnating material.

The extremely high power densities in the wire enamel coating arebelieved to be a principal factor in causing the coil failures duringaccelerated life tests of the ballast transformers using random woundnylon insulated wire. The power apparently dissipated in the wire enamelcoatlng raises the coil temperature above normal, and a thermal runawaytype of failure mechanism results. As the temperature increases, morepower is believed to be dissipated in the wire enamel coating withfurther increases in the coil temperature until a complete electricalbreakdown occurs.

It will be seen from a comparison of curves B and C that the powerdensities over the to 200 degree temperature range were considerablyless in the unimpregnated coil (curve B) than for the coil (curve C)that was impregnated. Thus, it was found that when coils without layerinsulation are impregnated with a material, the power. density in thedielectric system may increase sharply with temperature. It is believedthat the impregnating material provides additional paths for the flow ofcurrent between the turns of the random wound coil thereby increasingthe apparent power density in the insulating system.

In the bifilar coils and ballasts tested, the magnet wire had an outsidediameter of approximately 0.0143 inch. The average length of the contactwith an adjacent wire around the circumference of the conductor wire wasabout 0.001 inch. Thus, in an unimpregnated random wound coil a turn incontact with six adjacent turns, as shown in FIGURE 3, will make contactalong 0.006 inch of its 0.0450 inch circumference. However, in animpregnated random wound coil contact will exist through the impregnantbetween the total circumference of a turn and the adjacent six turns.Also, the impregnating material connects the pinholes and breaks in thewire covering. It Wlll be apparent, therefore, that the impregnatingmaterial provided an increased number of potential paths for.

current flow and introduced additional electrical losses in thecoil'circuit.

The foregoing tests in which bifilar coils were used to simulate randomwound coils, clearly demonstrated that the electrical characteristics atelevated'temperatures of a ballast coil without-layer insulation werecontrolling factors in the failure mechanism as contrasted to ballastcoils with layer insulation. Accordingly, measurements of the electricalcharacteristics of' ballast coils without layer insulation at anelevated temperature-were carried out.

In making the measurements :ofthe electrical characteristics-at elevatedtemperatures, it'was assumedtliat a bal'-- last coil can be representedby aparallel network as shown in FIGURE 1'. It can bereaclilydemonstrated 'that-a ballast coilexhibits an electrical behavoir that istyp'ical-ofa parallel RLC network. For example, if the start and finishleads of a ballast coil, before it 'is mountedon'a magnetic core, areconnected across an AC. voltage source, as the frequency of the voltagesource is increased from a relatively low value, the current through thecoil will decrease. Also, the phase angle between the voltage andcurrent'will' decrease.- When acertain critical frequency is reached,the current andvoltage will be in phase and the impedance of theball'ast'coil will be at its maximum value. Further, itwillbenotedlthatcurrent flowing through the coil will be at its minimumvalue. As the-frequency of theapplied voltage isfurther increasedaboveits critical value, it will be found'thatthe phase angle between thecurrent andvoltage Will-increase in magnitude. However, at values of thefrequency"above'the'critical value the current leads the appliedvoltage, whereas: at fre-' quencies below the critical frequency,current-willlag the. applied voltage. Therefore, the electrical behaviorof'a ballast coil is similar to that of'ra parallel RLC network.

The manner in which'the equivalent resistance R was determined 'willnow'be more fully described. The equivalent resistance Rat a frequencyof 60 cycles and at a specific temperature was obtained from thefollowing equation:

where X isthe reactance of the coils equivalent capacitance and D is thedissipation factorof this capacitance. It will be appreciated that=thedetermination of the value of equivalent resistance R-and themeasurements of other equivalent parameters, as described'herein; can beapplied to all fluorescent lamp ba-llastjcoils wound'withoutlayerinsulation. irrespective of the manner in which the coils are wound,random, precision or otherwise.

In order to obtain the capacitive reactance X the value. ofthe'equivalent capacitance C atroom temperature of a ballast coil was.first determined by resonating the coil with a beat frequencyoscillator to obtain the critical frequency of the coil. From the valueof the critical frequency, the capacitance of the coil wascomputed, andthen the value of capacitance. at: the critical :frequency was'converted to a value at 60 cycles per second, as will behereinafter'more fully explained.=-

It will be understood that the equivalent capacitance of the coil couldnot be convenientlymeasureddirectly at a frequency of 60' cycles persecond; The value of the capacitance at the critical frequency and atroom temperature was converted to a value atr 60 cycles per second bymultiplying the computed value at the critical frequency by aproportionality factor obtained by making measurements on a comparablereference'capacitor at both the critical'frequency and at 60Icycles .persecond, Similarly,

the value of capacitance of the coil at 60 cycles per second' wasconverted to a value at a desired elevated temperature T by aproportionalityfactor obtained by making measurements of the capacitanceof the reference capacitor at room temperature and at the desiredtemperature T;

Referring nowto FIGURE 8, I have illustrated therein the apparatus 65used to determine the'critical frequency of a ballast coil 66. It willbe seen that the ballast coil 66 to. be tested was connected across theleads 67,- 68"joined to the terminals of a beat, frequency oscillator69. A frequency counter 70 was also connected across the terminals ofthe beat frequency oscillator 69 to measure the frequency of the signal.applied across the ballast coil 66. The. ballast coil 66 was mounted on.a ferrite core 72 to increase the inductance of the coil 66 so that thecritical frequency of the coil 66 as mounted on the ferrite core 72'would fall within the frequency range of the beat frequency oscillator69. The ferrite core 72 may, of course, be.

eliminated Where the critical frequency of the coil is within thefrequency range'of the beat frequency oscillalator. 69.

Todetermine the critical frequency, the frequency of the signal suppliedby the beat frequency oscillator 69 was varied until a minimum voltagewas indicated on the scale of the voltmeter 71 connected in shunt withresistor R With a minimum voltage across the resistor R the current flowthroughthe resistor R is at a minimum therebyindicating a condition ofresonance. The frequency was then measured with the frequency counter 70by counting oscillations for one second intervals.

The inductanceL of the coil 66 mounted on the ferrite core 72 wasmeasured at 60 cycles per second and at room temperature. It was assumedthat this value would not significantly, change with temperatureorfre'quency. The measurements were made on a Hays and Maxwellinductance bridge circuit of. a General Radio type 1650-A impedancebridge.

Using the measured value of the inductance L obtained from'theimpedanc'ebridge' measurements, the capacitance C atthecriticalfrequency was computed from the following equation for thecritical frequency in cycles per second; f r

The procedure followed to convert the values of the capacitance C at thecritical frequency to the values at 60 cycles per second will now bemore fully described. As is shown in FIGURE 9, a reference capacitor wasformed by bringing out a start lead 61 and a finish lead 62 from theouter turn layer of coil 66. The finish lead 62 was soldered to thestart lead 61. A start lead 63 and a finish lead. 64 were brought outfrom a layer adjacent to the outer layer and' were connected as shown.Since the capacitance between the two turn layers'of the referencecapacitor changes in frequency in substantially the same manner as theequivalent capacitance of the coil 66, it was assumed that the values at60 cycles per second and at the critical frequency are proportional. ofthis reference capacitor was measured at room temperature and at thecritical frequency f of the ballast coil and at 'a'frequency of 60cycles pe'r secondwiththe General Radio impedance bridge used to measurethe inductance L, using the'beat frequency oscillator 69 as a source ofvoltage for the impedance-bridge. bridge was used -to measure thecapacitance of the reference capacitor at 60 cycles per second with thereference? capacitor being maintained at the desired elevated tem-'perature T. Further, the dissipation factor D at the desired elevatedtemperature T was measured with the impedance bridge;

To obtain the value of the equivalent capacitance of coil 66 at 60cycles per second at room temperature, the value-of the equivalent coilcapacitance at the critical frequency was multiplied by the ratio of thecapacitance of the reference capacitor at 60 cycles to the capacitanceof the reference capacitor measured at the critical frequency. The roomtemperature value of the equivalent capacitance of the coil 66 wasconverted to the value corresponding to the desired elevated temperatureT by multiplying the room temperature value by the ratio of thecapacitance of the reference capacitor at the desired temperature to thecapacitance of the reference capacitor at room temperature. Theequivalent capacitive reactance X was then computed from the followingrelationship:

1 X c where: fis the frequency in cycles per second, and

The capacitance Also, the impedance 1.1 C is the equivalent capacitanceof the ballast coil (at 60 cycles per second) in farads at the desiredtemperature T.

The value of the equivalent resistance R was determined from the valueof the dissipation factor D that was obtained simultaneously from themeasurements at 60 cycles per second of the capacitive value of thereference capacitor at the desired temperature T. When measurements aremade with the parallel capacitance comparison bridge circuit of aGeneral Radio impedance bridge, the values of the dissipation factor Dcan be obtained directly. Since the dissipation factor is a quantitythat is independent ofthe size of a capacitor and since the change inthe capacitance of the reference capacitor was essentially proportionalto changes in the capacitance of the ballast coil 66 with changes infrequency, the values of the dissipation factor D observed for thereference capacitor can be used to obtain equivalent resistance R of theballast coil 66. The values of the equivalent resistance R were computedfrom the following equation:

where:

X is the equivalent capacitive reactance of the coil at 60 cycles persecondand at the desired temperature T, and

D is the dissipation factor at 60 cycles per second for the referencecapacitor at the desired temperature T.

Having found the value of the equivalent resistance R corresponding to afrequency of 60 cycles per second and a temperature T, the power inwatts dissipated in the wire enamel of the coil was computed from theequation,

V2 P R where: V is the operating voltage of the coil, and R is theequivalent circuit resistance of the coil corresponding to a frequencyof 60 cycles per second and a temperature T.

. The volume of the wire enamel used in the coil 66 and the volume ofthe copper conductor were determined. The weight of the copper inkilograms was computed from the volume. The power was then expressed inwatts per cubic centimeter of wire enamel per kilogram of the copperconductor, and this value is referred to in this specification as thethermal runaway factor. I have found that this factor provides a basisfor predicting whether a ballast coil will fail as a result of a thermalrunaway condition developing within the coil.

- For the convenience of those desiring to practice the presentinvention, the following specific components were used in the apparatus65 shown in FIGURE 8 to determine the critical frequency of the ballastcoils tested:

Voltmeter 71 Hewlett Packard Model 400-A, Vacuum Tube Voltmeter, SerialNumber 14,572. Beat Frequency Oscillator 69 Bruel and Kjoer BeatFrequency Oscillator type 1014, Serial No. 22,687. Beckman Model 5230BPSerial No. 386.

Frequency Counter 70 ured value of the inductance L was found to be0.268

henry with a quality factor of 250. The value of selfcapacitance C wasthen calculated from the value of the resonant frequency f and theinductance L as follows:

(21rf L (21rX21,4-5O) X.268

C,,=2.06 10 farads The coil 66 was cut, and the leads 61, 62, 63 and 64were brought out and connected as shown in FIGURE 9 to provide theportion of the coil 66, which was used as the reference capacitor. Usingthe impedance bridge, measurements of the self-capacitance C if thereference capacitor were made at the resonant frequency of 21,500 cyclesper second, at 60 cycles per second, at room temperature (25 degreescentigrade) and at a temperature of degrees centigrade. The capacitanceof the reference capacitor at the resonant frequency and at roomtemperature was found to be 1.40 10- farads. At a frequency of 60 cyclesper second and at room temperature, the measured value was l.48 l0-farads. The value of the total equivalent circuit capacitance C wasconverted to a value at 60 cycles per second as follows:

A measurement was made of the capacitance of the reference capacitor at60 cycles an a temperature of 170 degrees centigrade. This value wasfound to be 2.64 10- farads. The dissipation factor D at 60 cycles andat a temperature of 170 degrees centigrade was found to be 5.67. Thetotal equivalent circuit capacitance at 60 cycles per second and roomtemperature was converted to the value C at a temperature of 170 degreesas follows:

equivalent circuit of coil 66 was determined from the followingequation:

To determine the volume of wire enamel in the coil 66, the diameter ofthe enameled wire was first measured and was found to be 0.1071centimeter. The wire enamel on a portion of the wire was removed with achemical stripper, and the diameter of the bare wire was found to be0.1023 centimeter. The total length of the wire was determined bymultiplying the mean length of a turn (17.0 centimeters) by the numberof turns (402) of the coil 66. The total length of wire was 6834centimeters. The volume of wire enamel (5.0 cubic centimeters) was foundby subtracting the volume of copper (56.2 cubic centimeters) from thevolume of the enameled wire (61.2 cubic centimeters). The weight of thecopper conductor was determined by multiplying the volume of copper bythe density in grams per cubic centimeter (8.92). The Weight of thecopper in the coil 66 was 0.500 kilogram.

r 13 The thermal runaway factor for'coil 66 was 0.0001491 wattdissipated in the equivalent circuit per cubic centimeterlof wire enamelper kilogram of copper.

InfTable I, I have listed the insulating systems used, and the thermalrunaway factor and the coil failures resulting from thermal runawaybased on temperature accelerated life tests of a group of ballasts usingthe coil constructions as indicated in Table I.

elongated central winding leg 88 on which the coil assemblies 84, 85 aredisposed and a pair of side yoke members 89, 90 which abut the oppositesides of the ends of the central winding leg 88 to form a closedmagnetic circuit. The highreactance of the transformer'87 is provided'tosome extent by the distributed leakage of the magnetic flux between theelongated central winding leg 88 and the side yoke members 89, "90.

Table I Results of Thermal Thermal Coil Insulating System Runaway TestsRunaway Factor Seeondary Nylon Wire Enamel Impregnated with Thermalrunaway 6.0

' Wax-Asphalt Mixture, 1,304 turns eviden 0.0320 dia. wire. Do;NylonWire Enamel Impregnated with .do 16. 4

Wax-Asphalt Mixture, 1,271 turns 00142 dia. wire. p

Do Nylon Wire Enamel No Impregnant, do 1.64

1,271 turns 0.0142 dia. wire. Do NylonWire Enamel, No Impregnant, Nothermal run- 0.6 1,304 turns 0.0320 dia. wire. away.

Do Formex Wire Enamel, Wax-Asphalt ,do '0. 01

'Irnpregnant, 1,271 turns 0.0142 die. wire.

I have found that a ballast coil for operating fluorescent lamps andWound without layer insulation will not fail prematurely by'a thermalrunaway mechanism if the thermal runaway factor, based on an equivalentresistance R'determined at a frequency of 60 cycles per second and atemperature between 140 and 180 degrees centigrade, was less than 1.6. g

In determining whether the ballast coil would fail as a result of athermal runaway condition, the coil was operated in a ballastelectrically loaded with appropriate lamps for the particular ballast inan elevated ambient temperature. The ambient temperature of the ovenwasadjusted to produce the desired average ballast coil temperatures asmeasured by the resistance rise of the coils. The time to failure attheelevated temperature was the time at which a fuse in the ballastprimary rated at approximately 1.5 times the normal current failed orthe ballast failed to operate the lamps.

Referring to FIGURES 10, 11 and 12, I have illustrated therein theimproved ballast coil arrangement of the invention embodied in a ballastapparatus 80 for operating apair of fluorescent lamps. In theperspective view of FIGURE 10, I have illustrated the apparatus 80 witha portion of the case 81 and potting material 82 cutaway to show theinternal disposition of the components, the electrical connectionshaving been omitted. The connec- ,tions are shown schematically in thecircuit diagram of FIGURE 12.

As will be seen in the cutaway portion of the primary coil assembly 84shown in FIGURE 10, no paper layer insulation was used. Also, thehigh'voltage or secondary coil assembly 85 was wound without layerinsulation. In the schematic circuit diagram of FIGURE 12 the equivalentresistance R and the equivalent capacitance C of the primary winding orcoil P are represented by a resistor and'a capacitor shown in dashedout-line. The equivalent inductance L is identified with the primarywinding P Similarly, the corresponding parameters of the secondarywinding or coil S are identified by the reference letters R C and L Inaccordance with the invention, the coils P and S were wound withoutpaper layer insulation. The parameters of the equivalent coil circuit, RC were such that values of the equivalent resistance R provided athermal runaway factor that was less than 1.6. A transformer 87 of theshell type was used to perform the voltage transforming and currentlimiting functions of the ballast apparatus 8 0. As will be seen inFIGURE 11, the transformer 87 includes the magnetic core 101 formed ofan Magnetic shunting pieces 91 may be provided bet-weenthecoil'assemblies '84, to'pro'vide a flux leakage path between thecoils'84, 85 if required. It will be understood that depending upon thedesign "of the transformer, the flux leak-age path or shunts may beformed either through nonmagnetic materials, such as air, or throughmagnetic material such as by insertable shunts or by shunt legs formedon the side yoke members'89 and 90.

The magnetic core 1010f the ballast transformer 87 is formed from astack of l-aminations' made ofmagnetic material. The laminations of thewinding leg .88 are stacked, and the coil assemblies .84, 85 are placedin as-' sem'ble-d relation on the center winding leg 88. The stacks oflaminations which form the yoke members 89, 90 are assembled with thecenter winding leg to form 'the magnetic core101 and are held inassembled relation by me-ans of clamps92 and 93 or may otherwisesecurely be heldtog ether by other suitable means.

As will be seen in the cutaway portion of the coil assembly 84 as shownin FIGURE 10, the conductor wire 86 is wound on a paper spool 95. Ifdesired, molded phenolic bobbins may be used. in place of the paperspool 95. Since coils P and S were wound on a spool 95 that did not havea rim to support the end turns, paper ground insulation 96 was providedto prevent the end turns from contacting the core laminations andgrounding. A paper wrapper 97 was provided on the outside of the coilassembly 84 to prevent the outer turn layer from being damaged duringhandling and assembling and also to insulate the outer turn layer. Asimilar wrapper 98 was provided for the coil assembly 85. It will beseen in FIGURE 10 that a two-section capacitor 98 is encased in thepotting material 82 and includes the series capacitor C and the startingcapacitor C of the ballast apparatus 80.

In FIGURE 12, I have illustrated a schematic circuit diagram showing theelectrical connections for the ballast apparatus 80 of FIGURE 10. Theballast transformer 87 must not only provide the requisite starting andoperating voltages for a pair of fluorescent lamps 1, 2 but must alsoprovide the ballasting action required to limit the current during thelamp arcing period. The ballasting action is necessary because the lampresistance characteristic-possesses a negative slope.

The ballast apparatus 80 as shown in FIGURE 12, includes the highreactance transformer 87, the series capacitor C and the startingcapacitor C Lamps 1 and 2 are positioned in proximity to a groundedconductive plate 100 so that the cathodes of the lamps 1 and 2 are 15-capacitively coupled with the conductive plate 100 to facilitatestarting. In most applications, the lighting fix- .ture in which lamps 1and 2 are mounted serves as the conductive plate 100.

In the schematic circuit diagram of FIGURE 12, it will be noted that thehigh reactance transformer 87 includes the magnetic core 101, theprimary winding P the secondary winding S cathode heating windings H H Hand the magnetic shunts 91. Secondary winding S is included in the coilassembly 85 shown in FIGURE 11. The cathode heating windings H H and Hare wound over the primary winding P and are included in the coilassembly 84 shown in FIGURE 11. If desired, the cathode heating windingsH H and H may be wound over a wrapper of paper insulation.

A pair of input terminal leads 102, 103 are provided for connection to asuitable alternating power source. When the ballast apparatus 80 isenergized, the cathode heating windings H H H supply the cathodes oflamps 1 and 2 with heating current. Cathode heating windings H and H areconnected in circuit with the lamps 1, 2 by output lead 104 and leads105, 106, 107. Cathode heating winding H which is an extension of theprimary winding P is connected in circuit with lamp 1 by output lead 108and lead 109.

When the input terminal leads 102, 103 are connected across analternating power source, the open circuit voltage developed across the,primary winding P and .secondary winding S is initially applied acrosslamp 1 because of the shunting action of the starting capacitor CFurther, during the open circuitcondition, to aid starting lamp 1 thecombined voltage across the primary winding P and secondary winding 8,,is applied between a cathode of lamp 1 and theconductive plate 100.After lamp 1 is started, it conducts current, and current flows throughthe starting capacitor C .Since lamp 2 is also disposed in closeproximity to the conductive plate 100, a starting aid potential is alsoapplied to a cathode of lamp 2 because of its capacitive coupling withthe conductive plate 100.

When both lamps 1 and 2 have started, the impedance of the startingcapacitor C relative to the lamp impedance is such that there isn-oappreciable current flow through the capacitor C Further, because ofthe impedance in .the capacitive coupling between lamps -1, 2 and theconductive plate 100, no significant current flow occurs between thelamps 1, 2 and the conductive plate 100 after the lamps are started.

By way of a more specific exemplification of the invention, a ballastapparatus was constructed for starting and operating two 96 PG 17 powergroove fluorescent lamps. The primary winding P was wound with 344 turnsof 0.0508 inch copper wire, and the secondary winding was wound with1300 turns of 0.0320 inch copper wire. The equivalent circuit parametersat 60 cycles and a temperature of 180 degrees centigrade of thesecondary coil assembly 84, were as follows:

Equivalent capacitance C 1.73 lmicrofarads Equivalent resistance R 3.5 Xohms For the secondary coil assembly the thermal runaway factor was0.0019. The ballast apparatus employing coils with these equivalentcircuit values satisfactorily passed temperature accelerated life testswith no evidence of thermal runaway. It was possible to contain theballast in a case having the following outer dimensions: 2% x 3%" x 13".A comparable ballast of the prior art would be housed in a case havingthe following outer dimensions: 2 /8" X 3% x 18". Thus, it will be seenthat significant reduction of 5 inches or approximately 28 percent ofthe length of the ballast was achieved by the practice of the'presentinvention.

It will be appreciated that to start power groove fluorescent lamps at atemperature between -20 and 90 degrees Farenheit, a ballast. mustprovide a peak starting aid voltage of 575 volts and R.M.S. voltage of500 volts. It will be noted that even though shorter lamps are used, thesame peak starting aid voltage must be provided by the ballast since thepeak voltage requirement of a fluorescent lamp depends upon thetemperature rating, if rcliable starting is required at a particulartemperature. Therefore, it will be appreciated that the ballasttransformers must supply appreciable voltages in order to start andoperate fluorescent lamps at lower temperature ratings. Despite theappreciable voltage requirement imposed upon the coils of a ballasttransformer, it is possible with the present invention to use coils insuch ballast transformers in which the layers of conductor wire arecontiguous or in other words, the layers of conductor wire are not woundover layers of insulation.

Turning now to FIGURE 13, I have illustrated therein a magnifiedsectional view of a portion of a conventional high voltage coil as seenunder a microscope. The insulated conductor wire turns are disposed inlayers and paper layer insulation 111 is interposed between the turnlayers. It will be apparent that the layer insulation 111 causes anappreciable displacement between the turn layers 110. This results in arelatively larger coil assembly for a given number of turns as comparedwith a coil wound with no layer insulation where the turn layers arecontiguous.

Heretofore, it has been generally considered necessary to provide paperinsulation between the layers of turns of a ballast coil to preventearly coil failures. Attempts made in the past to employ high voltagecoils without paper layer insulation were not successful because theballasts frequently failed, as a. result of a thermal runaway conditiondeveloping in the ballast coils.

It will be appreciated that in the typical ballast coil assembliespresently used to operate the smaller fluorescent lamps, such as the 40watt rapid start lamps, the open circuit voltage required is less than300 volts R.M.S. The. coil assemblies in the ballasts utilize a thinfilm nylon wire wound over a 0.002 inch vegetable parchment layerinsulation. These coils are not usually impregnated but are dipped in awax and asphalt mixture. The voltage between turns in the same layer isabout 0.10 volt and the voltage between layers is about 10 volts.

In view of the low voltage and voltage stresses present in such coils,the failures of coil assemblies without layer insulation in ballasttransformers have been attributed to various mechanisms involvingcopper-to-copper contacts in the finished coils. However, I havediscovered that a primary mechanism of failure in ballasts using coils-without layer insulation is essentially an electrical phenomenon.Further, I have found that if electrical parameters of the equivalentcoil circuit, such as the equivalent resistance, and the equivalentcapacitive reactance as determined at elevated temperatures aremaintained within certain limits to maintain the power dissipated in thewire enamel below a specified level as I have herein set forth, coilswithout layer insulation may be used in ballasts for operatingfluorescent lamps and will not fail as a result of a thermal runawaycondition. The coils may be random precision or universal wound as maybe desired in a particular application, so long as the criteria for theelectrical parameters in accordance with my invention are met. In otherwords, the thermal runaway factor or the power dissipated in the wireenamel in watts per cubic centimeter of the wire enamel per kilogram ofthe metallic conductor must be less than 1.6 based on an equivalentresistance of the coil determined at a frequency of 60 cycles per secondand a temperature range between and degrees centigrade.

An important advantage of the arrangement of the invention is thatappreciable reductions in the size, weight and amount of materials usedin a ballast apparatus are made possible. Further, as compared withconventional ballasts, the improved arrangement has, resulted insignificant reduction in the noise level of the ballast.

It will be apparent to those skilled in the art, that there are manydifferent types of insulating systems that may be used in the practiceof my invention. Accordingly, it it not intended to limit my inventionto the specific exemplifications which I have herein described byway ofillustration of the invention. While the present invention has beendescribed by reference to specific exemplification thereof, it is to beunderstood that other modifications may be made by those skilled in theart without actually departing from the invention. It is, therefore,intended in the appended claims to cover all such equivalent variationsthat come within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent in the UnitedStates is:

1. A ballast apparatus for operating a fluorescent lamp from analternating power source, said ballast apparatus comprising: a ballasttransformer having a magnetic core with an elongated winding leg andside yoke members, at least a pair of coil assemblies disposed on saidWinding leg, one of said coils including at least a primary windingadapted for connection to the power source and the other of said coilassemblies including at least a secondary winding, said secondarywinding and said primary winding being comprised of turns of conductorwire formed of a metallic conductor coated with an insulating enamel, atleast said secondary winding being comprised of turns of contiguouslayers of conductor wire, means including output leads for connectionwith the fluorescent lamp and for applying at least the voltage of saidsecondary winding across said output leads, said coil assembly with saidsecondary winding having an equivalent resistance R and an operatingvoltage V such that the power V /R in watts dissipated in the equivalentcircuit divided by the number of cubic centimeters of the insulatingenamel in the coil and divided by the number of kilograms of metallicconductor in the coil is less than 1.6, said value of the equivalentresistance R being determined at a frequency of 60 cycles per second anda temperature between 140 and 180 degrees centigrade.

2. A ballast apparatus for starting and operating fluorescent lamps froman alternating power source, said ballast apparatus comprising: a highreactance transformer having a magnetic core formed with an elongatedwinding leg and side yoke members, said side yoke members and saidwinding leg defining coil receiving windows, at least a pair of coilsmounted on said winding leg and disposed within said coil receivingwindows, one of said coils including at least a primary winding and theother of said coils including at least a secondary winding, said primarywinding and secondary winding being inductively coupled on said magneticcore, means including output leads for connection to the lamps wherebythe voltage across at least said secondary winding is supplied at saidoutput leads for operating said lamps, a pair of input leads forconnection with the power source, said primary winding being connectedin circuit with said input leads, said secondary winding and saidprimary winding being comprised of turns of conductor wire formed of ametallic conductor insulated with resin, at least said secondary windingbeing impregnated and wound to provide turns arranged in contiguouslayers, said coil including said secondary winding having an equivalentresistance R, and an operating voltage V such that the power V /R inwatts dissipated in the equivalent circuit divided by the number ofcubic centimeters of enamel in the coil and divided by the number ofkilograms of metallic conductor in the coil is less than 1.6, said valueof the equivalent resistance R being determined at a frequency of 60cycles per second and a temperature between 140 and 180 degreescentigrade.

3. A ballast apparatus for operating at least one fluorescent lamp froman alternating power source, said ballast apparatus comprising: aballast transformer having a magnetic core, a primary winding and asecondary winding inductively coupled with the primary winding on saidmagnetic core, a pair of input leads for connection to the alternatingpower source, said primary winding being connected across said inputleads, and means including output leads for supplying the output of theballast transformer across said at least one electric discharge lamp,said primary winding being included in a first impregnated coil woundwith contiguous layers of conductor wire, said secondary winding beingincluded in a second impregnated coil wound with contiguous layers ofconductor wire, said conductor wire being formed of a copper conductorcovered with an insulating enamel, each of said coils having anequivalent resistance R and an operating voltage V such that the power V/R in watts dissipated in the equivalent circuit divided by the numberof cubic centimeters of enamel in the coil and divided by the number ofkilograms of metallic conductor in the coil is less than 1.6, said valueof the equivalent resistance R being determined at a frequency of 60cycles per second and a temperature between and degrees centigrade.

4. A ballast apparatus for operating fluorescent lamps from analternating power source, said apparatus comprising: a high reactancetransformer having a shell type magnetic core, a primary winding and asecondary winding inductively coupled with said primary winding on saidmagnetic core, a pair of input leads for connection with the alternatingsource, said primary winding being connected across said input leads,means including a starting capacitor and output leads for supplying theoutput of the apparatus to. said lamps and including electrical leadsfor connecting said starting capacitor in shunt with one of said lamps,a series capacitor connected in series circuit relation with saidsecondary Winding, said circuit means including connections for placingsaid series capacitor in series circuit relation with said lamps, saidprimary winding being wound in a coil formed of contiguous layers ofconductor wire and disposed on said magnetic core, said secondarywinding being wound in a coil formed of contiguous layers of conductorwire and disposed on said magnetic core, said conductor wire beingformed of a metallic conductor coated with insulation, and means forinsulating said coils from said magnetic core, each of said coils havingan equivalent resistance R and an operating voltage V such that thepower V /R in watts dissipated inv the equivalent circuit divided by thenumber of cubic centimeters of conductor insulation in the coil anddivided by the number of kilograms of metallic conductor in the coil isless than 1.6, said value of the equivalent resistance R beingdetermined before said coil is assembled on said magnetic core at afrequency of 60 cycles per second and a temperature between 140 and 180degrees centigrade.

5. A ballast apparatus for operating at least one fluorescent lamp froman alternating power source, said ballast apparatus comprising: a highreactance ballast transformer having a shell type magnetic core, aprimary winding and a high reactance secondary winding inductivelycoupled therewith on said magnetic core, said primary winding beingadapted for connection across the alternating current power supply,circuit means including electrical leads for supplying the output of theapparatus to said at least one lamp, said primary and secondary windingsbeing wound with conductor wire to form coils without layer insulation,said conductor wire being comprised of a metallic conductor insulatedwith a resin enamel, each of said coils being impregnated with aninsulating material and having an equivalent resistance R and anoperating voltage V such that the power V /R in watts dissipated in theequivalent circuit divided by the number of cubic centimeters of theinsulating enamel in the coil and divided by the number of kilograms ofmetallic conductor in the coil is less than 1.6, said value of theequivalent resistance R being determined before said coil is assembledon said magnetic core at a frequency of 60 cycles per second and atemperature between 140 and 180 degrees centigrade.

6. A coil assembly for use in a ballast transformer having a shell typemagnetic core and adapted for operating fluorescent lamps, said coilassembly comprising: a spool of insulating material formed with an axialopening adapted for mounting on the magnetic core of the ballasttransformer and at least one winding formed of a plurality of turns ofconductor wire wound on said spool without layer insulation andimpregnated with an insulating material, said conductor wire beingformed of a metallic conductor insulated with a resin enamel, said coilhaving an equivalent resistance R and an operating voltage V such thatthe power V /R in watts dissipated in the equivalent circuit divided bythe number of cubic centimeters of the insulating enamel in the coil anddivided by the number of kilograms of metallic conductor in the coil isless than 1.6, said value of the equivalent resistance R beingdetermined at a frequency of 60 cycles per second and a temperaturebetween 140 and 180 degrees centigrade.

7. A coil assembly for a ballast transformer adapted for operating oneor more fluorescent lamps, said coil assembly comprising: at least onewinding formed of a plurality of turns of conductor wire, said conductorwire being formed of a copper conductor insulated with a resin enamel,said winding being random wound to form a coil with an axial opening andimpregnated with a resin, an insulating means disposed within said axialopening to insulate the coil from the magnetic core of the ballasttransformer, said coil having an equivalent resistance R and anoperating voltage V such that the power V /R in watts dissipated in theequivalent circuit divided by the number of cubic centimeters of theinsulating enamel in the coil and divided'by the number of kilograms ofmetallic conductor in the coil is less than 1.6, said value of theequivalent resistance R being determined at a frequency of cycles persecond and a temperature between and degrees centigrade.

8. A coil' assembly for mounting on a winding leg of shell type magneticcore of a ballast transformer for operating fluorescent lampscomprising: a spool of insulating material formed with an axial openingand adapted for mounting on the winding leg of the magnetic core, a coilincluding at least one Winding comprised of a plurality of turns ofconductor wire wound on said spool without layer insulation, saidconductor wire being comprised of a copper conductor and coating ofinsulating enamel, said coil being impregnated with a resinous materialand having an equivalent resistance R and an operating voltage V suchthat the power V /R in watts dissipated in the equivalent circuitdivided by the number of cubic centimeters of the insulating enamel inthe coil and divided by the number of kilograms of metallic conductor inthe coil is less than 1.6, said value of the equivalent resistance Rbeing determined at a frequency of 60 cycles per second and atemperature between 140 and 180 degrees centigrade.

References Cited by the Examiner UNITED STATES PATENTS 2,971,124 2/1961Feinberg et a1 315257 X 3,141,112 7/1964 Eppert 315-257 X 3,200,2908/1965 Moerkans 31599 GEORGE N. WESTBY, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner.

4. A BALLAST APPARATUS FOR OPERATING FLUORESCENT LAMPS FROM ANALTERNATING POWER SOURCE, SAID APPARATUS COMPRISING: A HIGH REACTANCETRANSFORMER HAVING A SHELL TYPE MAGNETIC CORE, A PRIMARY WINDING AND ASECONDARY WINDING INDUCTIVELY COUPLED WITH SAID PRIMARY WINDING ON SAIDMAGNETIC CORE, A PAIR OF INPUT LEADS FOR CONNECTION WITH THE ALTERNATINGSOURCE, SAID PRIMARY WINDING BEING CONNECTED ACROSS SAID INPUT LEADS,MEANS INCLUDING A STARTING CAPACITOR AND OUTPUT LEADS FOR SUPPLYING THEOUTPUT OF THE APPARATUS TO SAID LAMPS AND INCLUDING ELECTRICAL LEADS FORCONNECTING SAID STARTING CAPACITOR IN SHUNT WITH ONE OF SAID LAMPS, ASERIES CAPACITOR CONNECTED IN SERIES CIRCUIT RELATION WITH SAIDSECONDARY WINDING, SAID CIRCUIT MEANS INCLUDING CONNECTIONS OF PLACINGSAID SERIES CAPACITOR IN SERIES CIRCUIT RELATION WITH SAID LAMPS, SAIDPRIMARY WINDING BEING WOUND IN A COIL FORMED OF CONTIGUOUS LAYERS OFCONDUCTOR WIRE AND DISPOSED ON SAID MAGNETIC CORE, SAID SECONDARYWINDING BEING WOUND IN A COIL FORMED OF CONTIGUOUS LAYERS OF CONDUCTORWIRE AND DISPOSED ON SAID MAGNETIC CORE, SAID CONDUCTOR WIRE BEINGFORMED OF A METALLIC CONDUCTOR COATED WITH INSULATION, AND MEANS FORINSULATING SAID COILS FROM SAID MAGNETIC CORE, EACH OF SAID COILS HAVINGAN EQUIVALENT RESISTANCE R AND AN OPERATING VOLTAGE V SUCH THAT THEPOWER V2/R IN WATTS DISSIPATED IN THE EQUIVALENT CIRCUIT DIVIDED BY THENUMBER OF CUBIC CENTIMETERS OF CONDUCTOR INSULATION IN THE COIL ANDDIVIDED BY THE NUMBER OF KILOGRAMS OF METALLIC CONDUCTOR IN THE COIL ISLESS THAN 1.6, SAID VALUE OF THE EQUIVALENT RESISTANCE R BEINGDETERMINED BEFORE SAID COIL IS ASSEMBLED ON SAID MAGNETIC CORE AT AFREQUENCY OF 60 CYCLES PER SECOND AND A TEMPERATURE BETWEEN 140 AND 180DEGREES CENTIGRADE.